Cook onion with salt and pepper until soft, about 15 minutes. Stir in sorrel and cook over moderate heat, uncovered, about 5 minutes. Stir occasionally so the sorrel is evenly cooked.

Add half and half, water, and Tabasco pepper sauce. Bring to a boil and remove from heat. Puree in a blender until smooth and strain through a medium sieve. Stir in lemon juice, adjust with salt and pepper, and refrigerate a minimum of 4 hours, or, to serve immediately, chill in bowl rested in another bowl of iced water.